The present invention relates to an electrostatic capacitance sensor, and more particularly to the electrostatic capacitance sensor in which a conductive rubber is abutted against electrodes covered with a resist, thereby to vary electrostatic capacitance of the conductive rubber, and to change output level with the variation of the electrostatic capacitance.
Referring to FIGS. 4 to 7, a related electrostatic capacitance sensor of this type will be described. In FIG. 4, a switch device 1 includes a pair of first electrodes 3 mounted on a base plate 2, and a second electrode 4 in a C-shape as seen in a plan view which is circumferentially arranged around the first electrodes 3 so as to be apart therefrom. The first electrodes 3 and the second electrode 4 are covered with a resist 5, and a pair of carbon electrodes 6 are mounted on an upper face of the resist 5 and above the first electrodes 3.
Moreover, there is provided a click rubber 7 above and opposed to the first electrodes 3 and the second electrode 4. The click rubber 7 has a substantially cylindrical body 8 and a flexible leg 9 extending downwardly from an outer peripheral part of the cylindrical body 8 at a lower end thereof. In addition, a conductive rubber 10 substantially in a disc-like shape is provided in a center part of the lower end of the cylindrical body 8. The conductive rubber 10, the first electrodes 3 and the second electrode 4 constitute an electrostatic capacitance sensor 13. Further, a button 11 is mounted on an upper face of the click rubber 7. An upper cover 12 for covering an upper part of the click rubber 7, the first electrodes 3 and the second electrode 4 is provided to press down at a bottom parts of the flexible leg 9 of the click rubber 7, so that an upper part of the button 11 is projected through an opening 12a which is formed in an upper part of the upper cover 12.
FIGS. 5A and 5B show particulars of the first electrodes 3 and the second electrode 4. The first electrodes 3 are provided in a center of the C-shaped second electrode 4 so as to be apart from each other. The carbon electrodes 6 are mounted above the first electrodes 3 which are covered with the resist 5. Lead lines 14 are respectively connected to the carbon electrodes 6, and the one end of the one lead line 14 is grounded. Moreover, a lead line 15 is connected to the second electrode 4. A two dotted chain line in FIG. 5A represents a range of an area upon which the conductive rubber 10 is brought into contact.
FIG. 6 shows a circuit 16 of the switch device 1. The circuit 16 includes a clock power source 17, resistors 18, 19, an ENOR gate 20 (also known as an exclusive NOR or XNOR gate), and an electrostatic capacitance sensor 13. The electrostatic capacitance sensor 13 is composed of a capacitor 21 and a switch 22 connected in series. The resistors 18, 19 are respectively connected to the clock power source 17. The other end of the resistors 18, 19 are respectively connected to two input terminals of the ENOR gate 20. Then, one end of the electrostatic capacitance sensor 13 is connected to the resistor 18 and the ENOR gate 20, and the other end of the electrostatic capacitance sensor 13 is grounded.
In this state, the capacitor 21 corresponds to a circuit constituted by the conductive rubber 10, the first electrodes 3 and the second electrode 4, while the switch 22 corresponds to a circuit constituted by the conductive rubber 10 and the carbon electrodes 6.
Incidentally, when the button 11 is depressed, the flexible leg 9 of the click rubber 7 is flexed so that the conductive rubber 10 of the click rubber 7 is brought into contact with the carbon electrodes 6 to establish electrical conduction between the carbon electrodes 6. As the button 11 continues to be pushed down, the conductive rubber 10 is pushed against the first electrodes 3 and the second electrode 4 which are covered with the resist 5. According to the pressure contact, the electrostatic capacitance in the conductive rubber 10 is varied so that output from the conductive rubber 10 is thereby changed.
In other words, after the switch 22 has been turned on in the circuit 16, the output of the ENOR gate 20 is changed in accordance with the variation of the electrostatic capacitance of the capacitor 21.
FIG. 7 is a graph showing relationship between pressure of the electrostatic capacitance sensor 13 provided by bringing the conductive rubber 10 into contact with the resist 5 and the output level therefrom. As shown by a solid line, the output level rises substantially in proportion to a rise of the pressure.
However, in FIG. 7, there exists dispersion in the output level for the pressure as shown by dotted lines so that the output levels are deviated from an ideal output levels as shown by solid lines. The dispersion may incur instability of the output level of the electrostatic capacitance sensor 13 so that reliability as the electrostatic capacitance sensor 13 may be deteriorated.
It is therefore an object of the present invention to provide an electrostatic capacitance sensor in which the dispersion of the output level for the pressure of the electrostatic capacitance sensor is decreased in order to stabilize the output level thereof.
In order to achieve the above object, according to the present invention, there is provided an electrostatic capacitance sensor comprising:
a base board;
an electrode, formed on the base board;
a resist, which covers the electrode;
a conductive rubber, including:
a first face having a first area, which is opposed to the resist; and
a second face which is opposed to the first face, the second face having a second area which is larger than the first area; and
a flexible click rubber, on which the second face of the conductive rubber is attached, for providing pressure contact of the conductive rubber with respect to the resist.
Preferably, the first face and the second face are parallel with each other, and a sectional area of the conductive rubber perpendicular to a direction in which the pressure contact is provided, is made minimum at the first face.
Here, it is preferable that a form of the conductive rubber is shaped into a frustum.
Preferably, a form of the conductive rubber is shaped into a frustum of a circular cone.
Preferably, a peripheral end portion of the first face of the conductive rubber is chamfered.
According to the present invention, there is also provided an electrostatic capacitance sensor comprising:
a base board;
an electrode, formed on the base board;
a resist, which covers with the electrode;
a conductive rubber, which is shaped into a semispherical shape; and
a flexible click rubber, on which a flat face portion of the semispherical conductive rubber is attached, for providing pressure contact of the semispherical conductive rubber with respect to the resist.
In the above configurations, dispersion in the output levels generated with a deformation of the conductive rubber of the electrostatic capacitance sensor can be decreased, and stabilization of the output levels can be attained.